The present invention relates to an exposure method and apparatus for exposing a resist film to extreme ultraviolet (EUV) radiation through a photomask.
As semiconductor devices for a semiconductor integrated circuit have been downsized, it has become increasingly necessary to further reduce the feature size of a line-and-space pattern. And to define a fine-line pattern, a lithography technique is indispensable. Particularly when a pattern with a line width of 0.07 μm or less should be defined, a lithography technique using EUV radiation with a wavelength of around 13 nm as an exposing radiation is expected to be very effective.
In a known lithographic process using krypton fluoride (KrF) excimer laser radiation (with a wavelength of around 248 nm) or argon fluoride (ArF) excimer laser radiation (with a wavelength of around 193 nm), an exposure process is carried out in the air or nitrogen ambient. However, if the same exposure process is performed in such an ambient using EUV radiation, then the radiation is absorbed into oxygen or nitrogen molecules contained in the ambient, because the EUV radiation has a much shorter wavelength. This is why the EUV exposure process should be carried out in a vacuum.
FIG. 6 schematically illustrates a cross-sectional structure for a known EUV exposure apparatus. As shown in FIG. 6, a substrate holder 2 is placed on the bottom of a vacuum chamber 1 to hold a semiconductor substrate 4, on which a resist film 3 has been formed, thereon. On the ceiling of the vacuum chamber 1, a mask holder 6 is placed to hold a reflective mask 5, in which a desired mask pattern has been defined, thereon.
An EUV radiation source 7 is disposed on the vacuum chamber 1. The EUV radiation emitted from the EUV radiation source 7 is reflected off from a reflective mirror 8 toward the reflective mask 5, reflected again by the reflective mask 5 and then passed through a reflection/demagnification optical system 9 to impinge onto the resist film 3. The image formed on the resist film 3 has had its size reduced to ⅕, for example. In this manner, the mask pattern defined in the reflective mask 5 is transferred onto the resist film 3.
FIG. 7 illustrates the flow of a known process for defining a resist pattern out of a chemically amplified resist material.
First, in Step SB1, a resist material is applied onto a semiconductor substrate to form a resist film thereon. Next, in Step SB2, the resist film is pre-baked to vaporize a solvent contained in the resist film.
Then, in Step SB3, the resist film is exposed to EUV radiation, thereby transferring the pattern of a reflective mask onto the resist film. Subsequently, in Step SB4, the resist film is post-baked so that the acid diffuses in the exposed or non-exposed parts of the resist film.
Finally, in Step SB5, the resist film is developed using an alkaline developer, thereby defining a resist pattern.
When a resist pattern should be formed out of a normal (or non-chemically-amplified) resist material, the resist film is exposed to EUV radiation and then developed immediately without being post-baked.
The present inventor carried out the known exposure process of transferring the mask pattern from the reflective mask (i.e., photomask) 5 onto the resist film 3 repeatedly. As a result, I found that the EUV radiation exposure dose of the resist film 3 decreased gradually, thus deteriorating the reproducibility or accuracy of the pattern actually formed.